Telecommunications equipment is typically designed for indoor temperature controlled environments, but is now frequently deployed in outdoor locations. When telecommunications equipment is deployed in outdoor locations, a cabinet is typically used to provide environmental protection and control thermal conditions within the design limits of the telecommunications equipment. The electronic equipment is generally contained in one or more removable card modules that plug into a backplane in the cabinet. The card modules are supported in the cabinet by a subrack and include connectors that mate with corresponding connectors on the backplane. Cooling air flows through the subrack between the card modules to cool the electronics.
An EMI shielded card module, such as one for a wireless communications base station, typically includes a base, a circuit board assembly, and a cover. The secondary side of the circuit board assembly typically rests on the base. There are ground traces plated or etched onto the primary side of the circuit board, which divide the board into sections. The sections are populated with various components. The ground traces are not covered with solder mask, but are left exposed. The cover has walls with locations corresponding to the ground track locations on the primary side of the circuit board. The walls create cavities in the cover. When the cover is placed on the circuit board, each cavity covers a section on the circuit board, creating a compartment that is EMI shielded from the other compartments. Between the cover walls and the ground traces is an electrically conductive gasket to provide electrical contact across the interface between the cover's walls and the exposed ground traces, thereby creating the EMI shield. Screws or other fasteners are used to secure the cover, gasket, and circuit board to the base.
Such card modules are vulnerable to environmental contamination, such as moisture, salt, and other pollutants. For instance, in a salt fog environment, salt and moisture come into contact with the electronics through various routes. One primary route is through areas that are difficult to seal, such as around connectors that may protrude through the base or the cover. Another route is by wicking through very tiny spaces between the screw threads and the base, even when the screws are properly seated and tightened. Still another route is through the gasket seal after it has been attacked and weakened by the exposure to salt fog.
Sealed cabinets are sometimes used to protect the sensitive electronics from the environmental contaminants. Sealed cabinets use closed heating and cooling systems to maintain the electronics in the cabinets within the desired temperature range without exposing the electronics to potentially harmful contaminants. The heating and cooling systems include air conditioners, heaters, and/heat exchangers which consume space in the cabinet and add considerably to the cost of the cabinet. These components also require periodic maintenance to maintain them in proper operating condition.
Ventilated cabinets provide an alternative to sealed cabinets. Ventilated cabinets use natural or forced convection to draw ambient air through the cabinet to cool the equipment inside the cabinet. Ventilated cabinets are less expensive than sealed cabinets, consume less space, and are more easily maintained. However, the electronics on both the card modules and the backplane are typically exposed to the airflow, which may contain environmental contaminants such as moisture, nitrates, hydrocarbons, sulfur dioxide, nitrogen oxides, hydrogen sulfides, chlorine, ozone, salt, and the like.
Some protective methods have been developed, such as the method shown in U.S. Pat. No. 5, 527,989 to Leeb, which describes an encapsulation method to protect a circuit board from environment contaminants. However, using the Leeb encapsulation method does not protect connectors or any other components that require visual or physical accessibility, and is therefore not appropriate for many applications.
Alternatively, the card module itself may be sealed against intrusion of the environmental agents. One sealing approach is to hand caulk known gaps. Another approach is to use custom gasketing or custom connectors. However, both these methods are expensive and/or require extensive hand operations.
Another common sealing approach is to conformally coat the circuit board(s) of the card modules. That is, the circuit board and all of its components are completely encapsulated inside of a very thin layer of material that does not affect the function of the circuitry. The material is sprayed, dipped, painted, deposited, or otherwise applied to cover the exposed surfaces of the board and the associated electrical components (except external connector contacts).
One problem with conformal coating of shielded modules concerns the ground traces used for shielding. For proper EMI shielding, it is important for these ground traces to remain bare so as to insure proper electrical contact with the gasket. That is, when the circuit board is conformal coated, the ground traces must not be insulated from the gasket. If they are, the ground traces do not contact the gasket and the EMI shield is not properly formed. Under the prior art, the ground traces would be masked prior to the application of the conformal coating. After the conformal coating process, the masking would be removed from the ground traces leaving them free of coating. Next, the cover, gasket, board, and base would be assembled.
The method of individually masking each ground trace prior to coating and then unmasking after coating requires careful manual labor. The intensive use of surgical blades and delicate handling necessary to cut the masking away from the coated areas makes this method expensive. In addition, since the conformal coating is cut and disturbed during the unmasking process, the desired result expected by conformal coating, i.e., a completely coated surface, may not be met. In particular, the cut areas are potential weak points that the environmental conditions faced by the equipment during deployment, especially salt fog, can attack.
Another problem is that even with conformal coatings, connectors are still subject to attack by corrosive agents. In order to insure proper electrical contact at the connector pins, the connector pins should not be coated. Therefore, both the ground traces and connector contacts need to be masked during the coating process to prevent coating of these elements. Without coating, water or other agents may seep through the joint between the connector parts into the connector pin area and degrade the electrical connection. While water-proof connectors are known, these are generally more expensive and increase the cost of the equipment.
Thus, there remains a need for a simple and economical approach for protecting circuits within electronic enclosures from harmful environmental agents.